MAP Technical Reports Series No. 106 UNEP

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representing about 50 to 60% of the Mediterranean bound basin. To use these figures for nutrient
load estimates it is assumed that the export coefficients for nitrogen and phosphorus calculated
for the 4 Italian rivers plus the Rhône are true for the whole Mediterranean. The respective river
nitrogen and phosphorus load estimates are listed according to the 10 Mediterranean basins
defined by UNEP (cf. Table 14).
While the individual figures appear to be reasonably correct for rivers of the Northern
arc, they are very uncertain for rivers of other regions. Difficult to assess are figures from
countries that drain only partially into the Mediterranean and/or having large badly drained
unproductive areas (e.g., wadies). Therefore, the single estimates may not be too precise, but
the respective totals, excluding the Nile, would amount to about 1 million t/y for nitrogen, and to
0.14 million t/y for phosphorus.
Further the effect of large irrigation systems, as installed in several Mediterranean
countries, on nutrient export is difficult to assess without actual measurements. Thus, it is
impossible to say what the effects on nutrient discharges to the Mediterranean of the Asswan
High Dam construction, the closure of one Nile arm, and the changes in agricultural practice of
Egypt have been. The highly developed millennium old irrigation system that drains mostly into
the Northern delta lakes acts largely as sinks for phosphorus and nitrogen. On the other hand,
the Egyptian use of fertilizers has dramatically stepped up over recent decades (cf. Table 9).
Elster and Vollenweider (1961) and Vollenweider and Samaan (1972), studying L. Mariout, L.
Edku and the Nousha Hydrodrome found this latter to be still oligo-mesotrophic by 1957-59, while
Mariout receiving untreated city waters from part of Alexandria was already hypertrophic. On the
other hand, the trophic conditions of Nousha Hydrodrome, which receives land drainage waters,
have meanwhile strongly deteriorated. Thus it is possible that though the average water
discharge of the Nile has decreased substantially, concentrations have increased, but probably
not to the point of balancing the former nutrient load.
Regarding total loads, a source of uncertainty are direct discharges into marine waters
from coastal cities and municipalities that are not draining into major rivers. On the other hand,
the estimated totals of nutrients from river discharges are reasonably in agreement with the
above model estimates, which, taken at face value, would largely confirm the figures obtained
with different methodologies.
The UNEP (1984) estimates for 1976. Based on very scanty data, a UNEP expert group
that met twice in 1976, estimated a nitrogen load from resident population and rivers discharging
to the Mediterranean of 800,000 to 1,200,000 t/y, and correspondingly a phosphorus load of
260,000 to 460,000 t/y. While the fractional load estimates of the coastal population (44 millions)
+ industry and agriculture ( ca. 200,000 t/ of nitrogen; 57,000 t/y of phosphorus) are reasonably
comparable to the way of our estimates, the indirect load estimates that would result from the
river drained hinterland are likely to be on the low side for nitrogen (600,000 to 1,000,000 t), but
clearly in excess for phosphorus (200,000 to 400,000 t) compared to our estimates. The
respective average river N/P load would be around 2.5 to 3, which in no way is supported by
direct estimates and measurements. Accordingly, also the exchange load estimates between
the Mediterranean and the Atlantic through the Strait of Gibraltar made by Béthoux (cf. below),
are questionable.
d) Aeolian depositions
Beside nutrient supply to marine waters by rivers and direct discharges, aeolian
deposition of nitrogen, phosphorus and other inorganic and organic trace species transported
by air currents from land locked sources has increased over recent decades over the